Nanomechanical resonator can be used as an amplifier for measurements in quantum physics

Physicists at the Low Temperature Laboratory of Aalto University have shown how a nanomechanical oscillator can be used to detect and amplify feeble radio waves or microwaves. A measurement using such a tiny device, resembling a miniaturised guitar string, can be performed with the least possible disturbance. The results were recently published in the British journal Nature. The Low Temperature Laboratory is part of the Academy of Finland's Centre of Excellence in Low Temperature Quantum Phenomena and Devices.

The researchers cooled the nanomechanical oscillator, a thousand times thinner than a human hair, down to a low temperature near absolute zero at -273 °C. Under such extreme conditions, even near macroscopic-size objects follow the laws of quantum physics, which often contradict common sense. In the Low Temperature Laboratory experiments, the nearly 1 billion atoms comprising the nanomechanical resonator were oscillating in step in their shared quantum state.

The scientists had fabricated the device in contact with a superconducting cavity resonator, which exchanges energy with the nanomechanical resonator. This allowed amplification of their resonant motion. This is very similar to what happens in a guitar, where the string and the echo chamber resonate at the same frequency. Instead of the musician playing the guitar string, the energy source was provided by a microwave laser.

Microwaves amplified by the interaction of quantum oscillations

Researchers from the Low Temperature Laboratory showed how to detect and amplify electromagnetic signals almost noiselessly using a guitar-string like mechanical vibrating wire. In the ideal case, the method adds only the minimum amount of noise required by quantum mechanics.

The presently used semiconductor transistor amplifiers are complicated and noisy devices, and operate far away from a fundamental disturbance limit set by quantum physics. The scientists showed that by taking advantage of the quantum resonant motion, injected microwave radiation can be amplified with little disturbance. The principle hence allows for detecting much weaker signals than usually.

"Any measurement method or device always adds some disturbance. Ideally, all the noise is due to vacuum fluctuations predicted by quantum mechanics. In theory, our principle reaches this fundamental limit. In the experiment, we got very close to this limit," says Dr Francesco Massel.

"The discovery was actually quite unexpected. We were aiming to cool the nanomechanical resonator down to its quantum ground state. The cooling should manifest as a weakening of a probing signal, which we observed. But when we slightly changed the frequency of the microwave laser, we saw the probing signal to strengthen enormously. We had created a nearly quantum-limited microwave amplifier," says Academy Research Fellow Mika Sillanpää, who planned the project and made the measurements.

Certain real-life applications will benefit from the improved amplifier based on the new method, but reaching this stage requires more research efforts. Most likely, the mechanical microwave amplifier will be first applied in related basic research, which will further expand our knowledge of the borderline between the everyday world and the quantum realm.

According to Academy Research Fellow Tero Heikkilä, the beauty of the amplifier is in its simplicity: it consists of two coupled oscillators. Therefore, the same method can be realised in basically any media. By using a different structure of the cavity, one could detect terahertz radiation, which would also be a major application.

The devices used in the measurements were fabricated by VTT Nanotechnologies and microsystems. The research was funded by the Academy of Finland, the European Research Council (ERC) and the European Union.